Process for making a plate for a golf club head face, and golf club head

ABSTRACT

A process for making a plate composed of a β-type titanium alloy material for a golf club head face includes subjecting the β-type titanium alloy material to cold working with controlling the reduction rate thereof to vary the reduction rate depending on a position across a plain direction of the β-type titanium alloy material, and then subjecting the β-type titanium alloy material to an aging treatment so as to build up a hardness distribution across the plain direction of the β-type titanium alloy material.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No. 2003-161069, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for making a plate composed of a β-type titanium alloy material for a golf club head face having a hardness distribution with different hardness values across the face, and a golf club head using such a plate.

2. Discussion of the Background

Titanium alloys have been hitherto widely used as materials for airplane and automobile parts and medical instruments, as well as consumer goods such as eye glass frames and leisure equipment such as golf clubs and bicycles, because they exhibit high strength in spite of their relatively small specific gravity. Specifically, Ti-6Al-4V is widely used as a typical titanium alloy, but a cast product of this alloy, when used in a golf club head, poses problems of increasing a manufacturing cost and hardly reaching a required strength due to a complicated shape of the golf club head.

There is a known technique in order to address the above problem, in which a β-type titanium alloy material having a cold workability is subjected to cold plastic deformation so as to increase its strength for use in a golf club head. Japanese patent application laid-open no. 2001-54595 (a prior reference 1) discloses the use of a metal material produced by using a cold forging technique with a cold working ratio of 15% or higher in combination with an aging technique. This prior reference mentions that the use of the metal material as disclosed produces a desirable effect to increase the strength of a face-forming metal plate throughout its entire region in order to ensure a high material resistance against impact applied to a golf club head, thus achieving durability against crack of the material.

Japanese patent application laid-open no. 2001-231895 (a prior reference 2) discloses that a golf club head with a high performance to improve spring-like effect to a golf ball can be provided by building up hardness distribution across a surface of a metal plate mounted to a ball striking face so as to have a hardness higher in a center region and lower in a peripheral region. In this reference, a metal plate is partially subjected to heat treatment so as to have this hardness distribution.

Japanese patent application laid-open no. 2000-5354 (a prior reference 3) teaches a technique to build up different bending strengths in center and peripheral regions of a golf club face by changing the number of times at which forging is performed to each of these regions.

According to a conventional technique as disclosed in the prior reference 1, a plate composed of a β-type titanium alloy material produced by this technique, when used as a material for a golf club face, meets the durability requirements, but has less effect in improving spring-like effect to a golf ball and improving stability in carry of a golf ball, for which a demand is rapidly growing in recent years.

Contrarily to the above reference, the prior reference 2 discloses that the hardness of a club face (i.e. a ball hitting surface) is gradually decreased from the center region towards the peripheral region across the face, aiming at improvement of spring-like effect. This teaching was studied by the present inventors in various aspects and found out that while a need exists to build up an optimum hardness distribution with high precision as desired by a designer throughout the face of a metal material in order to produce the above effect for a golf club face, the prior reference 1 discloses only the possibility to achieve change in hardness of a metal material in a certain region by partial application of heat treatment, not teaching a specific technique for the change in hardness across the club face. Particularly for a plate of a club face having a width and height of only about 50 mm-150 mm, it is very hard to build up an intended hardness distribution in such a small area with high precision by a partial heat treatment. That is, it is unlikely to build up an originally designed hardness distribution on the metal plate, while preventing uneven temperature distribution and gradient.

The prior reference 2 neither teaches nor suggests a specific value of the difference in hardness to be applied for a desirable spring-like effect, so that it is hard to find out a proper surface hardness distribution for a product of a club face and a manufacturing condition for it.

The invention of the prior reference 3 is achieved mainly by providing different bending strengths in a material. However, a bending strength is primarily to be measured in a specific direction along which a material having a certain thickness and length is bent. Whilst, for a club face having a three-dimensional thickness distribution and curved shape, the bending strength cannot be accurately measured or determined. Even taking a certain point on the same plane, a value of the bending strength may be varied depending on the bending direction. Accordingly, even by the teaching of the prior reference 3, it is hard to give a mechanical characteristics to a club face so as to improve the spring-like effect while enhancing the stability in carry of a golf ball.

As described above, there is not hitherto disclosed a certain characteristics to be imparted to a plate product and a means for achieving improved spring-like effect of a club face by building up a certain mechanical characteristics distribution to the club face while enhancing the stability in carry of a golf ball. Thus, there is a great demand for an invention to achieve the above objects.

Accordingly, it is an object of the present invention to provide a process for making a plate composed of a β-type titanium alloy material for a golf club head face having a surface hardness distribution with the hardness varied across the face as intended, and a golf club head using this plate.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a process for making a plate composed of a β-type titanium alloy material for a golf club head face, including subjecting the β-type titanium alloy material to cold working with controlling the reduction rate thereof to vary the reduction rate depending on a position across a plain direction of the β-type titanium alloy material, and then subjecting the β-type titanium alloy material to an aging treatment so as to build up a hardness distribution across the plain direction of the β-type titanium alloy material. The β-type titanium alloy material, to which the cold working is subjected so as to vary the reduction rate depending on a position across the plain direction, can have a desired strain distribution across the surface after the cold working. Also, the β-type titanium alloy material with a desired strain distribution, to which the aging treatment is further subjected, can have a hardness distribution across the surface as exactly intended by a designer. In a case where the cold working is carried out by press forming, it is possible to control the plate thickness distribution as well as the reduction rate distribution by changing the shape of the press die. Accordingly, a material stiffness, which influences on the spring-like effect in a club, is possible to be obtained as desired, with the hardness distribution and the plate thickness distribution precisely built up across the face plate as designed. Thus, a plate suitable for a club face can easily be manufactured.

In the above process, the β-type titanium alloy material may be subjected to a solution annealing prior to the cold working. The solution annealing prior to the cold working enables the residual strain to be entirely removed in the history of working so that the β-type titanium alloy material can have a precisely controlled residual strain after the cold working and therefore have the hardness distribution more precisely built up across the surface after the aging treatment.

Moreover, in the above process, the cold working may be controlled so as to allow the β-type titanium alloy material to have a reduction rate varied across the plain direction with a minimum value of less than 10% and a maximum value of 35% or higher, and the aging treatment may be carried out at a temperature in the range of 300° C. to the β-transus temperature for 1-60 minutes. With this process, it is possible to allow the β-type titanium alloy material to have a proper reduction rate distribution for assuring the hardness required to a golf club face plate and have a sufficient hardness for a region to be formed with high hardness.

Moreover, the hardness distribution may be built up so as to have a Vickers hardness varied in the range of 200-500 across the plain direction while having a difference of 30 or greater between a maximum value and a minimum value in the Vickers hardness. A golf club head with the β-type titanium alloy material made in this process can enhance the spring-like effect of a golf ball and have an excellent stability in carry of a golf ball.

The Vickers hardness of the plate for a club face in this embodiment is measured according to a Vickers hardness measurement method defined by JIS Z 2244, in which the Vickers hardness is measured at five points substantially equally spaced to each other along the thickness and an average value calculated from them is herein designated as the Vickers hardness.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, and other objects, features and advantages of the present invention will become apparent from the detailed description thereof in conjunction with the accompanying drawings wherein.

FIG. 1 is a flowchart illustrating each step of processing according to an example of the present invention.

FIG. 2A is a plan view illustrating a shape of a material used in the example before pressure is applied. FIG. 2B is a cross section taken along lines A-A in FIG. 2A.

FIG. 3 is graphs illustrating the hardness distribution of face plates of the example of the present invention and a comparative example.

FIG. 4 illustrates club heads fabricated by respectively using the face plates of the example of the present invention and the comparative example, illustrating how the coefficient of restitution in each club head is distributed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Now, the description will be made for an embodiment of the present invention.

According to the present invention, a plate for a golf club face is composed of a β-type titanium alloy material, to which cold working is subjected with various reduction rates across the surface thereof, and then aging treatment is subjected, thereby making a plate for a golf club face with a desired hardness distribution across the surface.

A β-type titanium alloy material used in the present invention may have a varying composition. For example, it can be cited a β-type titanium alloy material having the following composition (wt. %): V 15-25, Al 2.5-5, Sn 0.5-4, O 0.12 or less, and Ti and inevitably contained impurities constitute the residue (as disclosed in Japanese Patent No. 2669004), or V 10-25, Al 2-5, Cr 2-5, Sn 2-4, O 0.25 or less, and Ti and inevitably contained impurities constitute the residue (as disclosed in Japanese Patent No. 264015). Of these compositions, it is preferable to use the β-type titanium alloy material having the following composition (wt. %): V 15-25, Al 2.5-5, Sn 0.5-4, O 0.12 or less, and Ti and inevitably contained impurities constitute the residue, because it exhibits excellent product strength and formability.

Any cold working technique such as cold rolling and cold forging may be used as long as it can impart strain to a plate so as to achieve different cold working ratio across the plate plain direction. For example, it can be cited a technique, in which a β-type titanium alloy material with concave and/or convex surface profile applied thereto by machining is prepared and then the surface profile of the β-type titanium alloy material is pressed into a desired shape.

An example of the present invention is presented for a more detailed description.

FIG. 1 is a flowchart illustrating each step of testing process of this example. As a β-type titanium alloy, Ti-20V-4Al-1Sn alloy was used. According to the cold working technique used in this example, the alloy plate is subjected to machining to have a certain surface profile to prepare a test piece, which is pressed into a flat shape. FIGS. 2A and 2B illustrate a test piece, that is, a material shape before subjected to pressing. According to the aging treatment as used in this example, the plate is subjected to the aging treatment in a salt bath for 15 minutes. In this aging treatment, a temperature rise period is about 20 seconds, and a cooling period is about 5 seconds in a cooling process initiated immediately at the time when the test piece is placed out of the salt bath.

FIG. 3 illustrates the hardness distribution of the plate composed of the titanium alloy plate for a golf club face prepared according to the process of this example. For determining the hardness, a specimen was cut from the prepared plate, and the Vickers hardness was measured at 5 points of a specific region of the plate along the thickness from the opposite surfaces of the specimen (i.e., points respectively 0.1 mm away from the opposite surfaces, points respectively ¼ thickness away from the opposite surfaces, and the center of the thickness of the plate), and an average value calculated from them was designated as a value of the specific region.

In a comparative example, the same titanium alloy was used, in which it was subjected to cold rolling to have a substantially uniform strain across the plain direction (reduction rate: 20%).

As illustrated in FIG. 3, it is found that the plate of the comparative example after subjected to the aging treatment has a hardness substantially kept at a substantially constant value of about 360-420 (Hv) although some fluctuations are caused, while the plate of the present invention has a hardness varied from about 240 (Hv) to about 410 (Hv).

In a performance evaluation test for specimens of the example of the present invention and the comparative example, both were welded to driver heads to prepare golf club heads. The coefficient of restitution (COR) was measured at each region of the club face. The measuring of this COR generally follows a measuring method adopted by the United States Golf Association (“USGA”), which is widely used to obtain a standard regulating the spring-like effect of a golf club head. The measuring method of the COR adopted by the USGA is carried out by crushing a golf ball against a club head secured on a block at a given speed and measuring a maximum rebound speed at the time of the crush with a sweet spot of the golf club.

In this example, with reference to the method of the USGA, the impact point of the club face with the golf ball was displaced by a given distance every time the impact, with the golf club secured on the block and the ratio between a pre-impact speed Vin and a post-impact speed Vout at each impact point was measured so that the distribution thereof was determined. Contour maps prepared for the respective golf clubs based on the result of the measuring by using those specimens are illustrated in FIG. 4.

As apparent from the illustrated contour maps, a golf club head to which the titanium alloy plate prepared in the example of the present invention is applied has not only a higher COR than a golf club head of the comparative example, but also a much greater area of the higher COR than the same.

The test results conducted under various cold working and aging treatment conditions are shown in Table 1, in which the reduction rate across the surface was varied in the cold working by varying the thickness (t1, t2) of the plate of FIG. 1. The face plates, all having a thickness of 2.6 mm, were respectively welded to driver heads with a head volume of 400 cm³ and a loft of 10.5°, for which the investigation was made on durability and spring-like effect. For the evaluation of the durability of each golf club face, a hitting test was conducted, in which a golf ball was hit 2000 times with a head speed of 55 m/sec, so that club faces with and without visually observed cracks or depressions were evaluated as pass (O) and fail (X).

For the evaluation of the distribution of the COR, a club face, of which a region with a COR of 0.82 or more occupies 40% or more of the face area, was evaluated as pass (O) and otherwise, evaluated as fail (X). TABLE 1 (TEST RESULTS) Thickness Reduction rate Distribu- Distribu- Hardness tion tion Distribution Max. Min. Aging (Hv) Distribution NO. Value Value Treatment Min. Max. Durability of COR 1 30% 30% 450° C. × 20 380 390 ◯ X min. 2 50% 50% 450° C. × 20 420 435 ◯ X min. 3 35% 10% 280° C. × 60 230 235 X Not min. Measured due to Depression 4 35%  6% 350° C. × 60 240 420 ◯ ◯ min. 5 35%  3% 450° C. × 20 235 385 ◯ ◯ min. 6 35%  0% 550° C. × 3 min. 215 330 ◯ ◯ 7 35%  0% 800° C. × 3 min. 220 240 X Not Measured due to Depression 8 60%  5% 450° C. × 20 241 450 ◯ ◯ min. 9 75%  5% 450° C. × 20 235 465 ◯ ◯ min. 10 95%  0% 450° C. × 20 — — X Non-usable min. due to cracks

It can be found from the result shown in Table 1 that when the maximum reduction rate in the cold working exceeds 90%, excessive strain is caused and cracking or another problem frequently occurs by the aging treatment in the next process. Therefore, the maximum reduction rate in the cold working is preferably set to 90% or lower.

With the aging treatment at a temperature below 300° C., a plate is hard to be aged even if the residual strain in the cold working exists. On the other hand, at a temperature above the β-transus temperature, a plate may be forced to be solution treated, making it hard to apply strain and then aging treatment to the plate in the subsequent heat application. Therefore, it is preferable to set the temperature of the aging treatment in the range of 300° C. to the β-transus temperature. A plate with the aging treatment subjected for less than one minute is hard to be aged. On the other hand, a plate to which the aging treatment was subjected for over 60 minutes is entirely aged and therefore is hard to have different values of the hardness distributed across the plain direction. Thus, the aging treatment is preferably controlled to continue for 1-60 minutes.

As described above, according to the process for making a plate for a golf club head face, it is possible to build up a hardness distribution across the plain direction of the plate as intended by controlling the reduction rate in the cold working. Also, in a case where the cold working is carried out by press forming, it is possible to control the plate thickness distribution as well as the reduction rate distribution by changing the press die. Accordingly, a material stiffness, which achieves an intended spring-like effect of a golf ball when used for a club face, is possible to be obtained with the hardness distribution and the plate thickness distribution precisely built up to a face plate as designed. Thus, a club face with high performance can easily be manufactured. In this regard, according to the present invention, it is not necessary to allow only the center region of the face plate to have high hardness. Alternatively, plural regions with high hardness may be provided across the surface, so that an optimum hardness distribution can be built up as desired by a designer in such a small region of a golf club face plate.

The golf club head of the present invention has an excellent durability, as well as producing an excellent spring-like effect and enhanced stability in carry of a golf ball.

This specification is by no means intended to restrict the present invention to the preferred embodiments set forth therein. Various modifications to the process for making a plate for a golf club head face and a golf club head, as described herein, may be made by those skilled in the art without departing from the spirit and scope of the present invention as defined in the appended claims. 1.3 

1. A process for making a plate composed of a β-type titanium alloy material for a golf club head face, comprising subjecting said β-type titanium alloy material to cold working with controlling the reduction rate thereof to vary the reduction rate depending on a position across a plain direction of said β-type titanium alloy material, and then subjecting said β-type titanium alloy material to an aging treatment so as to build up a hardness distribution across said plain direction of said β-type titanium alloy material.
 2. The process for making a plate according to claim 1, wherein said β-type titanium alloy material is subjected to a solution annealing prior to said cold working.
 3. The process for making a plate according to claim 1, wherein said cold working is controlled so as to allow said β-type titanium alloy material to have a reduction rate varied across the plain direction thereof with a minimum value of less than 10% and a maximum value of 35% or higher, and said aging treatment is carried out at a temperature in the range of 300° C. to the β-transus temperature for 1-60 minutes.
 4. A golf club head comprising said plate manufactured according to claim 1, wherein said hardness distribution is built up so as to have a Vickers hardness varied in the range of 200-500 across the plain direction of said β-type titanium alloy while having a difference of 30 or greater between a maximum value and a minimum value in said Vickers hardness. 